1. Field of the Invention
The present invention relates to a liquid crystal display and a driving method thereof, and more particularly, to a liquid crystal display in which acoustic noise, which is caused by common voltage signals driven in a form of alternating current (AC), is removed and a driving method thereof.
2. Description of the Related Art
As information oriented society has been developed, demands for displays for displaying an image are increasing. Thus, various flat displays such as a liquid crystal display (LCD), a plasma display panel (PDP), and organic light emitting display (OLED), and a vacuum fluorescent display (VFD) have been used recently.
The LCD among the flat displays is generally implemented in an active matrix type using thin film transistor (TFT) devices. The LCD is widely used now because of the advantage of a small size and thickness, light weight, and low power consumption.
The LCD includes two substrates facing each other and a liquid crystal disposed between the two substrates. The LCD displays an image by changing arrangement of the liquid crystal by an electric field generated between a pixel electrode and a common electrode that are formed on the two substrates.
As shown in FIG. 1, a liquid crystal panel 51 includes a first substrate 52 and a second substrate 53 that face each other, and a liquid crystal 54 interposed between the substrates 52 and 53. In the first substrate 52, TFTs and pixel electrodes are formed. In the second substrate 53, a color filter and the common electrode are formed.
The liquid crystal panel 51 has liquid crystal cells (pixels) that are regions defined by scan lines and data lines, and the pixels are disposed in a form of a matrix (two-dimensional array). The reorientations of molecules of the liquid crystal are controlled in every liquid crystal cells so that the image is displayed in the liquid crystal panel 51.
The reorientations of the liquid crystal molecules within the liquid crystal cells are controlled by a voltage applied between an electrode formed in the second substrate 53 (a common electrode) and pixel electrodes of the first substrate 52. The application of the voltage is controlled by turning on/off the TFT that is formed in each of the liquid crystal cells.
Moreover, the LCD is driven considering reliability of the liquid crystal material, that is, to prevent deterioration of the liquid crystal material. An alternating current (AC) type voltage signal, in which polarity of voltage alternates with time, is applied to the liquid crystal material, which is formed in each of the pixels, for every time periods.
For the methods of driving the LCD using the AC type voltage signals, there are a line reversing method, a source reversing method, and a dot reversing method. Among the methods, the line reversing method reverses the polarities every low lines on the panel to apply an image signal to the respective liquid crystal cells.
That is, the line reversing method, for example, as shown in FIG. 2, is configured to reverse the polarities of the voltage applied to the liquid crystal cells by varying a voltage (depicted by a solid line in FIG. 2) applied to the common electrode by one horizontal period 1H and a voltage (depicted by a dotted line in FIG. 2) of the image signal applied to the liquid crystal cells.
As such, when the liquid crystal is driven by the AC drive, signals with inverse phases to each other are applied to a pair of electrodes (common electrode and pixel electrode) and a voltage (bias) is applied between the electrodes.
Therefore, the LCD is driven by the line inverse method so that the second substrate 53 in which the common electrode is formed vibrates in accordance with the application of the voltage to the common electrode.
At this time, since a drive frequency of the common electrode (a frequency of a voltage applied to the common electrode) is about 10 kHz at a liquid crystal panel for a current portable device, the second substrate 53 vibrates at about 10 kHz when driving the LCD.
Since the vibration has a frequency within an audio frequency band of human being (20 Hz to 20 kHz), the vibration is recognized by a user as sound to be harsh to the ear, that is, a noise. Such noise is seriously rising as a series problem as a thickness of a portable device employing the LCD becomes thin and a distance between the LCD and the portable device is being narrow.